A relatively recent development in wireless communications involves one wireless communication device transmitting, at the same time on the same bandwidth, to multiple spatially separated destination wireless communication devices. Each destination wireless device receives one or more space-time streams (STSs). This is known as downlink multi-user multiple-input multiple-output (DL-MU-MIMO) communications, i.e. simultaneous transmissions from one device to many devices, such as one wireless access point (AP) to many wireless clients. MU-MIMO techniques are part of the IEEE 802.11ac wireless network communication standard. The multi-user nature is different from a single destination device receiving all of the spatial streams, according to the IEEE 802.11n standard, for example.
The transmitting wireless device, e.g., an AP, “precodes” the transmit signal streams in order to achieve certain objectives. Each destination device typically has fewer antennas and/or receive chains than the total number of space-time streams. Consequently, the destination device cannot perform conventional MIMO equalization on all the space-time streams and then pick out the destination device's own stream. Rather, the AP needs to suppress the space-time streams not intended for a particular destination device at the non-intended devices. This processing of the transmit signal streams is referred to as precoding. If the destination device has more antennas and/or receive chains than the space-time streams intended for it, then the destination device can use its additional antenna and receiver resources for “interference suppression” with respect to the space-time streams not intended for that destination device. When the transmitting device performs precoding, the destination device can improve upon the precoding through the use of interference suppression. In the atypical case, the destination device may have more antennas and/or receive chains than the total number of space-time streams, and the destination device can therefore perform conventional MIMO receive processing (equalization) and “pick out” its space-time stream(s). This is known as “interference cancellation.”
Interference suppression and cancellation can be improved if the transmitting device (e.g., the AP) sends training patterns for all space-time streams to all destination devices. The precoding operation attempts to achieve both (a) good signal strength of a space-time stream at the intended destination device (“beamforming”), and (b) near-zero signal of a space-time stream at a device that is not the intended recipient or destination device of that space-time stream (“null steering”). If both of these objectives are achieved, the signal-to-interference plus noise ratio (SINR) at the destination device can be improved. This is the overall objective of precoding: Obtain good SINR at each destination device so that each destination device can MIMO equalize (with interference suppression/cancellation as available) the signal and successfully recover the destination device's own space-time stream(s).
However, maximizing SINR is a complicated, nonlinear, iterative process. Zero Forcing and Minimum Mean Squared Error are known precoding methods, but are not readily suited to the case of an AP sending multiple space-time streams to a recipient when the number of space-time streams is less than the number of the destination device antennas and/or receive chains. Both Zero Forcing and Minimum Mean Squared Error involve selecting a subset of recipient antennas to target (where the subset has cardinality and is equal to the number of space-time streams destined for the recipient), and ignoring the remaining antennas of the destination device.